The present invention relates to the medical imaging arts. It particularly relates to high-speed and time-dependent helical or multi-slice volumetric cardiac computed tomography (CT) imaging, and will be described with particular reference thereto. However, the invention will also find application in volumetric computed tomographic imaging of other dynamically moving organs, in high resolution contrast agent intake and blood perfusion studies, and the like.
Cardiac computed tomography imaging typically employs an x-ray source that generates a fan-beam, wedge-beam, cone-beam or otherwise-shaped beam of x-rays that traverse an examination region within which a patient's heart is disposed. The cardiac tissue, coronary arteries, and blood interacts with and absorbs a portion of the traversing x-rays. Typically, a contrast agent is administered to the patient to improve blood contrast. A one- or two-dimensional radiation detector arranged opposite the x-ray source detects and measures intensities of the transmitted x-rays.
During scanning the patient is linearly advanced between axial scans to perform multi-slice computed tomography imaging, or the patient is continuously linearly advanced during x-ray source rotation to perform helical computed tomography imaging. The imaging data is reconstructed using a filtered backprojection, a PI reconstruction, or the like to generate volumetric image representations. Preferably, the cardiac cycle is monitored by an electrocardiograph or other device, and the imaging data is binned into cardiac phase bins to reconstruct the heart at a plurality of phases.
A wide range of cardiac studies are performed using cardiac computed tomography imaging. Qualitative review of cardiac computed tomography images by trained medical personnel detects congenital heart defects, large aneurysms or stenoses in the major coronary arteries, and other gross anatomical abnormalities. Analyses such as heart pumping capacity measurements, blood perfusion studies in coronary tissues, and coronary vessel tracking provide complementary quantitative diagnostic information.
In cardiac imaging, problems arise due to a limited temporal resolution of computed tomography, which is controlled by the rotation rate of the x-ray source. To reduce image artifacts, imaging data over at least a half-rotation of the x-ray source (i.e., 180° of data) is preferably acquired for each voxel. At presently achievable gantry rotation rates which are limited by x-ray flux, mechanical stability, and other factors, acquisition of a half-rotation of projection data requires about a tenth of a second or longer. Since the cardiac cycle spans about one second, substantial motion blurring is typically observed.
In cardiac cycle gating, imaging data is acquired using a circular or low-pitch spiral radiation source trajectory such that each voxel remains in the field of view over two or more cardiac cycles. Simultaneously acquired electrocardiographic data is used to select computed tomographic projection data from two or more cardiac cycles that approximately correspond to a selected cardiac phase. The selected data are combined to form a complete data set of about 180° or more for each voxel, and this combined data set is reconstructed to produce an image representation of that cardiac phase.
However, data combined from adjacent cardiac cycles may not readily form a complete data set due to angular redundancies. Synchronizing the rotation with the cardiac cycle to ensure angularly complementary data typically results in sub-optimal computed tomography imaging parameters, for example a reduced gantry rotation rate. Moreover the cardiac cycle can vary during image acquisition, especially in subjects with coronary disease or other cardiac malfunctions.
Another source of error with cardiac gating is inaccuracy in associating the electrocardiographic data with the cardiac cycle. It is known in the art that cardiac motion is only approximately related to the electrocardiographic signal, and that physical motion cycles of the heart components vary non-linearly with variations in the cardiac cycle period, and moreover vary from subject to subject. Particularly in cases of heart arrhythmia where the cardiac cycle period is sometimes variable over a few neighboring heart beats, simple linear scaling of cardiac cycle features with cardiac cycle period is of limited accuracy.
Yet another problem with cardiac gating is that for large volume fields of view the low-pitch spiral takes a substantial length of time to span the volume of interest. This can produce artifacts due to subject motion that vary in an unknown manner along the axial direction. Alternatively, a large-area beam and corresponding large-area detector can be employed to enable use of a larger spiral pitch. However, this increases system cost, and image artifacts can occur due to spatial non-uniformity of the large-area beam or detector.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.